The present invention relates generally to the use of Global Positioning System (GPS) sonobuoys to determine the location of re-entry vehicle (RV) impacts, and specifically to an automatic attenuator which helps distinguish between acoustic location signals and the leading-edge of the impact signals of (RVs).
The Soviet Union uses operational launch control centers to test launch their inter continental ballistic missile systems (ICBM's). The United States test launches its ICBM's from Vandenberg AFB, California where missiles are launched into the South Pacific near the Kwajalein Missile Range or one of several broad ocean area (BOA) target areas. Although the missiles have their own internal navigation systems, one of the purposes of the test launches is to determine the accuracy of the missile systems.
The task of accurately determining the impact points of RVs in the ocean at the time of splashdown is alleviated, to some degree, by the following references, which are incorporated herein by reference:
U.S. Pat. No. 3,461,421 issued to H. Stover on Aug. 12, 1968; PA0 U.S. Patent Application Ser. No. 666,784 filed on Oct. 31, 1984 by Edwin Westerfield; and PA0 U.S. Patent Application Ser. No. 377,214 filed on May 11, 1982 by Richard Lane.
The Stover reference discloses a sonobuoy system which has direction finding and range capabilities. The Westerfield and Lane references describe the use of a Global Positioning System to determine the geodetic location of a single sonobuoy in an array of sonobuoys and, in turn the determination of the splash point of re-entry vehicles.
This determination is usually made through use of the Sonobuoy Missile Impact Location System (SMILS). With this system up to 15 sonobuoys are dropped in an array in the target area. Each buoy has the ability of detecting acoustic (in water) noise generated by the splash of the re-entry body and of transmitting the re-entry data to an aircraft circling overhead. It is essential that the position of each sonobuoy be known.
To determine the location of other sonobuoys, with respect to each other, an aircraft command-actuated signal causes an acoustical pinger in each GPS sonobuoy to be activated. These GPS pings are received by the passive sonobuoys and transmitted to the aircraft via the VHF link for recording and processing. By knowing the time of GPS ping transmission and reception, as well as the speed of sound in water, the position of each passive sonobuoy can be determined by standard techniques. The speed of sound in water is obtained by measuring the ping transmission time between the GPS sonobuoys.
The GPS/SMILS system, as disclosed in the above-cited references, has been successfully used to determine the location of RV impacts in the broad ocean area (BOA). However, a variety of acoustic phenomena have been observed in the BOA which are either capable of being mistaken for an impact signal, or obscure the exact arrival time of the leading edge of the impact signal These other signals include bottom reflections, wave induced noise on the buoys, and noise spikes from unknown causes.
Static acoustic filtering is an inadequate solution to the problem. Although an impact signal has a comparatively high threshold in comparison to the other acoustic phenomena, currently employed sonobuoys still are required to detect acoustic pings to determine their location with respect to each other.
In view of the foregoing discussion, it is apparent that there currently exists the need to separate the leading edge of RV impact signals from other sonar acoustic phenomena while retaining the ability to detect GPS pings. The present invention is intended to satisfy that need.